It is sometimes necessary to install electrical equipment in a hazardous location where explosive gas may be present. If the electrical equipment creates a spark when explosive gas is present, an explosion can result. One method to prevent an explosion is to ensure that the explosive gas can never come into contact with the spark. In locations where explosive gas is present very rarely and for short periods, restricted breathing enclosures have been used to keep the explosive gas away from the sparking electrical equipment.
Restricted breathing enclosures may only be used if the average temperature of the air inside the enclosure may rise by a maximum of about 10° C. when compared with the ambient temperature of the air outside the enclosure, when all internal equipment is operating. In a traditional restricted breathing enclosure, the internal temperature rise may be kept below the maximum value only if the internal electrical equipment dissipated very low power. Any power generated by the internal equipment must be conducted through the wall of the enclosure. The rate of heat transfer must equal or exceed the rate of heat dissipation by the internal electrical equipment so that the average internal temperature rises less than the maximum.
Previously, restricted breathing enclosures were not used to enclose circuit breakers in hazardous locations where explosive gas might be present. Instead, other methods of protection such as explosion proof enclosures have typically been used for circuit breakers installed in hazardous locations. Circuit breakers dissipate significant heat, especially when they are operating near their trip point. This heat dissipation made it impractical to transfer the heat from the inside to the outside of the enclosure without exceeding maximum allowable internal temperature rise.